This invention relates to an improved expandable graphite gasket containing unexpanded or unexfoliated graphite which may be formed by mixing expanded and unexpanded graphite flakes and compressing the graphite material into a sheet or other shape as required by the joint. The gasket of this invention is particularly useful for forming sealed joints subject to pressure and heat, such as head and exhaust gaskets, which causes the gasket to expand and tightly seal the joint.
A conventional automotive gasket generally includes a graphite or paper lamina which is adhesively bonded or mechanically affixed to a steel core or metal plate. The most preferred automotive gaskets includes a metal plate having a plurality of spaced tangs struck from the plate, extending from the plane of the plate or core into the gasket material. All gaskets are subject to creep or extrusion of the gasket material during compression of the gasket by bolts or the like. Where tangs are not used, the gasket material will extrude laterally under pressure, reducing the torque retention of the gasket, thus causing leakage and reducing the life of the gasket. The tangs struck from the metal plate form "columns" between the opposed surfaces of the gasket, reducing lateral extrusion of the gasket material. The gasket material may be applied to one or both sides of the metal sheet or the gasket may be a laminate of several layers or laminae of metal and graphite or paper sheets. Generally, graphite sheets, when used alone, have too much creep for most automotive applications and thus the preferred gaskets include a metal plate or core.
Graphite gaskets are presently used by the automotive industry for exhaust and head gaskets, exhaust and intake manifolds, and the like. Graphite is a crystalline allotropic form of carbon. Graphite occurs naturally and is mined in several locations throughout the world, including for example, Madagascar, Mexico and China. Graphite is also produced synthetically by heating petroleum coke to approximately 3,000.degree. C. in an electronic furnace. Approximately 70% of the graphite presently used in the United States is synthetic. The naturally occurring graphite is processed by heating the graphite particulate or flakes to a temperature above the exfoliation temperature of the graphite (above about 300.degree. C.) which causes the graphite to "exfoliate" or separate and expand to several times their original thickness. The expanded or exfoliated flakes are then blown onto a conveyor, washed with a mild acid and compressed into sheets of various thicknesses. The sheets are then cut to size and used for various applications including gaskets, as described above.
A principle concern or problem with any gasket is potential leakage. Leakage may result from several sources including, for example, insufficient compression of the gasket or creep, as described above, and gaps or imperfections in the mating surfaces to be sealed. Further, automotive gaskets, for example, are subject to extreme variations in temperature. An automotive exhaust gasket, for example, is heated by the vehicle exhaust gases, which heat the gasket to temperatures greater than 400.degree. C., causing the gasket to expand. When the engine is "cold", however, the gasket tends to shrink, potentially resulting in leakage around the gasket. The gasket is also subject to creep, which is reduced by the use of tangs, as described above. Automotive manufacturers generally overtorque the bolts which secure the gasket between the mating surfaces about 10%, sometimes causing distortion, particularly around the bolts. Further, the surfaces to be sealed and the gasket are not perfectly matched and planar, particularly on a microscopic level. Generally, therefore, there are gaps and imperfections which may cause leakage. There is, therefore, a need for a gasket, particularly in automotive applications, which does not require overtorquing and which fills gaps and imperfections in the mating surfaces of the joint. The expandable gasket of this invention accomplishes these purposes.
The fact that naturally occurring graphite flakes exfoliate or expand several times their original size has been known for many years. More recently, Union Carbide Corporation has developed a use for sheets of graphite containing unexfoliated graphite in wall coverings and ceiling tiles as a flame retardant. When the expandable graphite layer is exposed to extreme heat, such as a fire, the expandable graphite exfoliates, absorbs oxygen and extinguishes the fire. Such material is available from UCAR Carbon Company, Inc. of Parma, Ohio. The advantages of the use of unexpanded or unexfoliated graphite has not, however, been suggested or recognized as a gasket material.